1. Field of the Invention
The present invention relates to a nitride semiconductor device employing a nitride semiconductor (for example, InaAlbGa1-a-bN, 0≦a, 0≦b, a+b≦1) used in a light emitting device such as a light emitting diode (LED) and a laser diode (LD), in a photodetector such as a solar cell and an optical sensor, and in an electric device such as a transistor and a power device.
2. Discussion of the Related Art
A nitride semiconductor device is utilized in a light emitting device such as a light emitting diode (LED) and a laser diode (LD), in a photodetector such as a solar cell and an optical sensor, and in an electric device such as a transistor and a power device. Especially, a nitride semiconductor has been widely utilized in an optical device such as a light emitting diode (LED) and a laser diode (LD). A nitride semiconductor device as an optical device generally has a double hetero structure in which an active layer made of a nitride semiconductor such as InGaN is placed between an n-side nitride semiconductor layer and a p-side nitride semiconductor layer each having a wider band gap than that of an active layer.
A variety of stacked layer structures have been considered for the p-side nitride semiconductor layer of the nitride semiconductor device having a double hetero structure. For example, Japanese Laid-Open Patent Application Publication No. 2000-183462 discloses a nitride semiconductor laser device in which at least one layer of a p-type electron confining layer made of Mg doped AldGa1-dN (0<d≦1), a p-type guide layer made of undoped GaN, a multi-layered p-type cladding layer including AlfGa1-fN (0<f≦1), and a p-type contact layer made of Mg doped GaN are stacked as a p-side nitride semiconductor layer on a multiquantum well active layer. In addition, a p-type guide layer made of undoped GaN contains Mg of 1×1016 cm−3 to 1×1018 cm−3 diffused from the p-type electron confining layer.
Japanese Laid-Open Patent Application Publication No. H11-251684 discloses a nitride semiconductor laser in which a p-side cap layer made of Mg doped AlGaN, a p-side cladding layer of superlattice structure with alternately stacked layers of GaN and AlGaN, and a p-side contact layer made of Mg doped GaN are stacked as a p-side nitride semiconductor layer on a multiquantum well active layer. In Japanese Laid-Open Patent Application Publication No. H11-251684, in order to reduce the threshold of the laser, the p-side cladding layer of a superlattice structure is initially grown undoped, and then grown so that the content of Mg therein gradually increases with the distance from an active layer. In addition, in the p-side cladding layer having a superlattice structure, the content of Al in the AlGaN layer gradually increases with the distance form the active layer. In addition, Japanese Laid-Open Patent Application Publication No. H11-177175 discloses a nitride semiconductor laser in which a p-side cap layer made of Mg doped AlGaN, a p-side optical guide layer made of undoped GaN, a p-side cladding layer of a superlattice structure with alternately stacked layers of GaN and AlGaN each having different impurity concentration, and a p-side contact layer made of Mg doped GaN are stacked on an active layer.
Japanese Laid-Open Patent Application Publication No. 2003-289176 discloses a GaN-based semiconductor laser in which an undoped InGaN optical waveguide layer, an undoped AlGaN cladding layer, an undoped InGaN layer, a p-type AlGaN electron blocking layer, a p-type AlGaN/GaN superlattice cladding layer, a p-type GaN contact layer are stacked as a p-side nitride semiconductor layer on a multiquantum well active layer. According to Japanese Laid-Open Patent Application Publication No. 2003-289176, the undoped InGaN optical waveguide layer, the undoped AlGaN cladding layer, and the undoped InGaN layer prevent Mg contained the p-type layers from diffusing into the active layer during crystal growth, aging, or the like.
Japanese Laid-Open Patent Application Publication No. 2004-112002 discloses a nitride semiconductor device in which a p-AlGaN/p-InGaN superlattice p-type layer and a Mg doped GaN/Si doped GaN modulation-doped p-side contact layer are formed as a p-side nitride semiconductor layer on a multiquantum well active layer. According to Japanese Laid-Open Patent Application Publication No. 2004-112002, the Mg doped GaN/Si doped GaN modulation-doped p-side contact layer improves a ESD tolerance (or electrostatic discharge protection) by reducing leakage current, and the p-AlGaN/p-InGaN superlattice p-type layer functions as a cladding layer and performs as a layer for confining light and injecting holes to the active layer. In addition, Japanese Laid-Open Patent Application Publication No. 2004-112002 discloses that the ESD tolerance can be increased if an AlGaN or GaN having a lower impurity concentration is formed between the p-AlGaN/p-InGaN superlattice p-type layer and the Mg doped GaN/Si doped GaN modulation-doped p-side contact layer.
However, in the conventional nitride semiconductor devices, it is needed that the ESD tolerance be further improved. The present inventors arrived at the present invention by considering that, in the nitride semiconductor devices, a decrease in the ESD tolerance occurs because of a local concentration in the electric current caused by a nonuniformity in the electric current flowing in the p-side nitride semiconductor layer, which is caused by the in-plane irregularity of the contact resistance between the p-side nitride semiconductor layer and the electrode, uneven distribution of the Mg concentration in the p-side contact layer, and the material and the shape of the electrode.
That is, although the p-side nitride semiconductor layer in the nitride semiconductor device shows the p-type conductivity through doping with a p-type impurity such as Mg, an activation rate of the p-type impurity in the nitride semiconductor is low so that it is difficult to reduce the resistance thereof. In addition, when the p-side nitride semiconductor layer is formed too thick, it may lead deterioration in the crystallinity and increase in the manufacturing cost so that there is a limitation in increasing the thickness of the layer. Therefore, sufficient in-plane distribution of the electric current in the p-side nitride semiconductor layer has been hardly obtained. Although it is possible to dispose an ohmic electrode with a wide area on the p-side nitride semiconductor layer so as to spread the electric current in the ohmic electrode, the contact resistance between the p-side nitride semiconductor layer and the ohmic electrode tends to be ununiform. In addition, a p-type impurity such as Mg is doped in a part of the p-side nitride semiconductor layer (p-side contact layer), to which the ohmic electrode being in direct contact, with a high concentration so as to establish a good ohmic contact, the p-type impurity concentration and the activation rate in the p-side contact layer may fluctuate in the plane of the p-side contact layer and the local irregularity may occur in the distribution of the electric current that flows in the p-side contact layer. Further, a ununiformity may occur in the electric current that flows in the p-side nitride semiconductor layer, depending on the shape of the ohmic electrode and the position and the shape of a pad electrode disposed on the ohmic electrode. As described above, in the conventional nitride semiconductor devices, an irregularity in the electric current flowing in the p-side nitride semiconductor layer occurs to cause a local concentration in the electric current, and the ESD tolerance is decreased.